It's not possible due to lots of Android/TWRP/recovery versions available. If you already have LE installed, you should be able to run reboot update or reboot recovery over SSH to boot from SD card or USB drive. You should also be able to upgrade in-place if using one of my LE 7 builds.
Thanks, I will try the reboot update. The reboot recovery boots into TWRP.
BTW, it is possible to flash u-boot besides the TWRP to allow SD/USB boot or these two occupy the same partition?
BTW2, I have flashed the lastest LE7 from here: LibreELEC 7.0 For S905/X - FreakTab.com